Two threads from replies here that are helpful: look at B2B SaaS or join enterprise IT. There are a lot of companies that are trying to modernize their tools and adopt agile ways of working, and if you're coming from a startup background you will bring a really valuable perspective and set of skills.
I know Fortune 500 developers who are working on super cool DevOps enablers and internal libraries using modern tech. The good places are not your dad's IBM WebSphere configuration shop anymore.
I worked for a good B2B SaaS, and while I was able to tell myself I'm not doing anything unethical or immoral, and while some of the customers were indeed very valuable (in terms of value to their customers) Fortune 500 companies - a large chunk of customers were adtech, dodgy mobile gaming, etc.
So you're still just providing infrastructure for these guys and not really contributing anything useful to the world, you're just one level removed from that, and hopefully you have enough customers who actually do good.
If the bar is "I don't want any code I write to be used for purposes which I do not personally endorse" then yes, it's probably a good idea to go work for a mission-driven non-profit and never make open source contributions to anything ever again
I know Fortune 500 developers who are working on super cool DevOps enablers and internal libraries using modern tech. The good places are not your dad's IBM WebSphere configuration shop anymore.